The present invention relates to three dimensional polygon antenna designs.
There are various types of antennas which are known in the art. These types include the full-wave (FW) square loop antenna, the rectangle class of antennas (of which the square is a member), the multi-loop antenna, and the large perimeter loop antenna which results from the use of thick wires in its construction.
The basic full-wave (FW) or 1 xcex loop element is a known antenna. A four-sided rectangular full-wave (FW) loop antenna can have many shapes, ranging from a folded dipole at one extreme to a 0.5 xcex transmission line which is terminated by two minute Hertzian dipole elements at the other extreme. The square variant was the first of this class to be developed.
FIG. 1 illustrates a square full-wave (FW) loop antenna, also known as a Quad loop. In FIG. 1, a square loop is shown that is nominally 1 xcex in perimeter, or 0.25 xcex per side. The design may be visualized as being comprised of two 0.5 xcex dipoles, separated in height by 0.25 xcex, with their ends folded down and touching. The antenna may also be visualized as illustrated in FIG. 2, which shows two U-shaped 0.5 xcex dipoles with truncated central radiating sections of 0.25 xcex and folded ends of 0.125 xcex.
Basically, then, rectangle-based antennas are comprised of two short parallel dipole radiating elements with the ends of the dipoles connected to each other by way of a pair of transmission line wires. The square is a unique case which has radiators and transmission line wires of equal size.
Only a single feedpoint is necessary and this port may be located at the center of any of the four sides. The polarization of the antenna depends on whether the feedpoint is placed at the center of a vertical or a horizontal wire. The wire opposite and parallel to the fed wire also radiates, because it carries in-phase and co-directional currents of equal magnitude.
The wires orthogonal to the radiators act as transmission lines, have a 180 degree phase shift at their centers (current minima), and carry currents which are anti-directional; therefore, for all practical purposes, these wires do not radiate. In the case of a square where only one radiating element is fed, the physical transmission line connections between the two radiators are necessary in order to maintain the proper phase relationships in the radiator currents.
In all subsequent discussions of rectangle-based antennas, the radiators will be referred to as including the fed wire and those elements which are parallel to it. xe2x80x9cTransmission line wiresxe2x80x9d will refer to the wires connecting the radiator ends to each other. The sizes of the radiators and transmission line wires are inversely related (i.e., as one lengthens the other must shorten) to maintain the overall loop perimeter somewhere at approximately 1 xcex.
The radiation resistance Rin of such a square loop is in the order of 120 ohms and is a product of the self resistance Rself of each of the truncated dipole radiators and the mutual resistance Rm induced by the parallel radiator. A close approximation of Rin for the square or any rectangular-shaped variant of a FW loop may be represented by the formula Rin=2(Rself+Rm).
These loops are only nominally 1 xcex in perimeter. Due to the capacitive reactance induced by the proximity of their high-voltage/low-current points at the centers of their transmission line wires (i.e., the points where the folded back dipole tips touch each other), the feedpoint reactance Xin of an antenna consisting of two such folded-down 0.5 xcex dipoles is highly negative. In order to resonate (Xin=0) such an antenna, the sides must be increased in size beyond 0.25 xcex and therefore the perimeter beyond 1 xcex.
The overall loop perimeter or length per side depends on the thickness of the wire composing the antenna: the greater the diameter of the wire, the greater the negative Xin and the greater the perimeter at resonance. With very thick wires (e.g., having diameters exceeding 0.03 xcex) the loop perimeter exceeds 1.3 xcex.
The bandwidth (BW) for all antennas discussed herein is defined by the standing wave ratio (SWR) 2:1 limits when referenced against Rin. This is also dependent on the wire thickness since it is a function of the Q-factor; the thicker the wire, the wider the bandwidth.
The square FW loop has broadside radiation from its two radiators, which are in phase and which have equal currents. It has a gain improvement over that of one of its constituent dipoles in the order of slightly more than 1 dB. This is due to the xe2x80x9cstacking effect,xe2x80x9d an aperture overlap between the radiation patterns of the two truncated dipoles separated by 0.25 xcex. Depending on the wire thickness, the overall gain of a square FW loop is in the order of 3.1-3.4 dBi.
There are two extremes in the shape of rectangular FW loops. One extreme is the folded dipole (FD), where the two radiators are almost 0.5 xcex long and the interconnecting transmission lines are minuscule. The Rin, derived from the formula above, is in the order of 288 ohms. The gain is that of a simple dipole but there is an improvement in the bandwidth.
The other extreme is that of two minuscule xe2x80x9cHertzian dipolexe2x80x9d radiators connected by a 0.5 xcex transmission line. The Rin approaches 0 ohms while the modeled gain, using Numerical Electronics Code (NEC), exceeds 6 dBi.
In between these two extremes in the shape of a FW loop, there are an infinite number of possible antennas with intermediate properties. The narrower the radiator, the lower the radiation resistance, the greater the gain, and the narrower the bandwidth. The gain is a function of the separation of the radiators. The bandwidth bears a direct relationship to the Rin. Other, well-known, antennas which function similarly are slot antennas.
Any FW loop can be attached directly to another. If two such loops are conjoined at a common radiator, a planar antenna results, with two equal-sized loops consisting of three radiators, as shown in FIG. 3. Since there are now three radiating elements with equal element separation, there is an increase in gain. NEC modeling of one of maximum gain, at the dimensional extreme of three Hertzian dipole radiators connected via 0.5 xcex transmission lines, yields a gain in excess of 7.1 dBi.
These double-loops have properties similar to the simple rectangles of which they are comprised. Their loop perimeters increase with wire diameter, their gain is a function of radiator separation, their radiation resistance is related to the self resistance of the radiators as well as the mutual resistance contributed by the two other parallel radiating wires, and their bandwidth is related to the input resistance.
There is a need for more compact antennas with improved broadband design capabilities and wider bandwidths, both in impedance and gain.
The antenna design of the present invention is a three-dimensional (3-D) arrangement of full-wavelength (FW) or 1 xcex loops. A major advantage of this antenna design is that it is very amenable to broadband design with a range of operating frequencies or bandwidth (BW) in excess of 3:1. Furthermore, the antenna design is relatively compact, with the maximum dimension being less than that of a half-wave dipole at the lowest frequency. The gain of this antenna design exceeds that of a dipole over the entire bandwidth by 1-1.5 dB. These antennas may be used by themselves or as individual elements in high-gain wideband arrays.
Variants of these antennas may also be designed for use as compact higher gain vertical scanning arrays where the beam pattern may be rotated electronically over a 360 degree azimuth. In addition to the increased forward gain, the other advantage in this type of use is in the very deep nulls off the rear which serve to minimize interference. Other variants, whether horizontally or vertically polarized, may be used for controlled squint of their elevation lobes.
With dimensional changes, these antennas can be designed to attain any feedpoint impedance in the range of 24-300+ ohms. In addition to the 3-D structure, the feed system is critical in achieving these performance objectives. This invention is for the family of 3-D rectangular-loop based antennas and for the feed system.
The present invention in one embodiment provides a three-dimensional antenna array, comprising a plurality of rectangular full-wave loops arranged in a three-dimensional array, comprising a plurality of radiators, each radiator having two ends, and a plurality of transmission lines, each transmission line connecting a pair of the radiators through a corresponding end of each connected radiator, wherein each of the plurality of radiators is fed from a common feedpoint substantially at a geometric center of the three-dimensional antenna array.
The present invention in another embodiment provides a three-dimensional antenna arranged as a cubic triangle, comprising three contiguous full-wave loops arranged vertically to form the cubic triangle, comprising three radiators, each radiator having two ends, and a plurality of transmission lines, each transmission line connecting a corresponding pair of the radiators through a corresponding end of each radiator in the connected pair of radiators, and wherein each of the three radiators is fed from a common feedpoint substantially at a geometric center of the cubic triangle.
The present invention in another embodiment provides a three-dimensional antenna array arranged as a cubic hexagon, comprising a first element having a first group of six radiators arranged in a hexagonal arrangement, and a first plurality of transmission lines, wherein each radiator in the first group of six radiators has two ends, and each transmission line in the first plurality of transmission lines connects a corresponding pair of radiators in the first group of six radiators through a corresponding end of each radiator in the connected pair of radiators; and a second element spaced apart from the first element and having a second group of six radiators arranged in a hexagonal arrangement and a second plurality of transmission lines, wherein each radiator in the second group of six radiators has two ends, and each transmission line in the second plurality of transmission lines connects a corresponding pair of radiators in the second group of six radiators through a corresponding end of each radiator in the connected pair of radiators, wherein a corresponding radiator in each element is fed in phase.
The present invention in another embodiment provides a three-dimensional antenna arranged as a cubic hexagon, comprising a group of six radiators arranged in a hexagonal arrangement, each radiator having two ends; and a plurality of transmission lines, each transmission line connecting a corresponding pair of radiators in the group of six radiators through a corresponding end of each radiator in the connected pair of radiators; wherein each radiator has a corresponding feedline extending to a common feedpoint substantially at a geometric center of the cubic hexagon.
The present invention in another embodiment provides an antenna, comprising a three-dimensional arrangement of full-wavelength loops having a plurality of radiators, each radiator being fed from a common feedpoint substantially at a geometric center of the three-dimensional arrangement.
The radiators and the transmission lines in each of the above embodiments may be orthogonal.